The present invention relates to a crucible of pyrolytic boron nitride, hereinafter referred to as PBN, useful for melting a source material in the molecular beam epitaxy as well as a method for the preparation thereof. More particularly, the invention relates to a PBN crucible useful as a source-melting vessel in the molecular beam epitaxy and capable of giving an epitaxial layer having a greatly decreased number of defects due to disordered molecular beam ejection from the crucible caused by adhesion of the melt in drops to the inner surface of the crucible.
The above mentioned molecular beam epitaxy is a technology under rapid progress as a promising method for the formation of a thin layer of which the structure can be controlled in a thickness of only a few atoms. For example, technologies relative to a so-called superlattice layer which is a controlled stratification of layers consisting of atoms of the elements such as aluminum, gallium and arsenic are under energetic development works for the application as a high-speed transistor because the electron mobility therein can be larger by several times under certain conditions than in a conventional epitaxial layer consisting of the same elements.
The molecular beam epitaxy is a method in which a layer is formed by the deposition of atoms in the form of molecular beams on a substrate surface while the molecular beams are ejected from a molecular beam source which is a melt of the source material in a molecular beam cell such as a so-called Knudsen cell consisting of a crucible for melting of the source material, heater, reflector and temperature sensor such as a thermocouple. The material of the crucible for the molecular beam epitaxy having the largest versatility is PBN in respects of the high purity, excellent heat resistance, high mechanical strength and so on although the material should be selected depending on the source material to be melted in the crucible.
When a source material such as gallium is melted in a source crucible made from PBN for molecular beam epitaxy, a trouble is sometimes encountered that the melt of the source material adheres in drops to the inner surface of the crucible along or in the vicinity of the opening periphery of the crucible so that the molecular beams emitted from the bulk of the melt are greatly disturbed by the beams emitted from the drops of the melt adhering to the crucible walls resulting in occurrence of a great number of defects in the epitaxially grown layer formed by the molecular beam epitaxy. This problem of disordered molecular beams is particularly serious at the moment when the drops of the melt adhering to the inner wall surface of the crucible fall off the wall and drop to the bottom of the crucible or to the melt contained therein.
As a countermeasure to solve the above mentioned problems, a method is proposed to have the inner surface of a PBN crucible finished by mirror-polishing (see Preprints for the 52nd Scientific Lecture Symposium of the Society of Applied Physics, No. 2, page 488, 12P-P-13). This method, however, is far from satisfactory because no reproducible results can be obtained by the mirror-polish finishing of the crucible surface alone.